[clang] [clang][ASTImporter] Invalidate ImportedTypes cache on Decl import failure (PR #214008)

[Balázs Kéri via cfe-commits]

balazske wrote:

The fix looks correct but I would rewrite all the comments because some non-technical language is used and for me the text is difficult to understand and not exact.

https://github.com/llvm/llvm-project/pull/214008